build, internal/build: misc improvements

[fjl]

This PR makes small adjustments to automated builds:

• darwin/386 builds don't make sense because macOS only runs on 64 bit machines.
• darwin builds should be distributed as tar.gz archives
• some scripts still referred to the godep workspace
• zip archives were not compressed. Adding compression shrinks the geth-only archive
  from ~20MB to ~6MB.

[mention-bot]

@fjl, thanks for your PR! By analyzing the history of the files in this pull request, we identified @karalabe, @tgerring and @obscuren to be potential reviewers.

[karalabe]

LGTM